THALLOPHYTES: FUNGI 



55 



Algse ; that the sexual spore is a zygote ; and that the two 

 pairing tip cells cut off from the main body by partition 

 walls are gametangia. Mucor^ therefore, is isogamous. 



ZT^Tm^^^ /^"^??^$ 



Fig. 38. Sexual reproduction of Mucor, showing tips of sex branches meeting {A), 

 the two gametangia cut oflf by partition walls {B), and the heavy-walled zygote 

 (C).— Caldwell. 



41. Peronospora. — These are the " downy mildews," very 

 common parasites on seed plants as hosts, one of the most 

 common kind attacking grape leaves. The mycelium is coeno- 

 cytic and entirely internal, ramifying among the tissues 

 within the leaf, and piercing the living cells with haustoria 

 which rapidly absorb their contents (Fig. 39). The pres- 

 ence of the parasite is made known by discolored and 



